OOTB checkbox is not available in AEM 6.3 metadata schema editor. But, it can be enabled and used with steps as described below.

Step 1: Overlay builditems.jsp to Enable checkbox

Create an /apps overlay for “/libs/dam/gui/coral/components/admin/schemaforms/formbuilder/builditems.jsp“

In “/apps/dam/gui/coral/components/admin/schemaforms/formbuilder/builditems.jsp“, uncomment the following snippet. It would be available between line 90-98

At this point, browse to a metadata schema editor:

Click on navigation rail. Browse to Tools > Assets > Metadata Schemas

Click on default > Select edit

Notice that checkbox is available in the menu.

Though you can currently add checkbox for a metadata schema, you will not be able to persist the checkbox state properly.

Step 2: Persisting checkbox state

Create an /apps overlay for “/libs/dam/gui/coral/components/admin/metadataeditor/clientlibs/metadataeditor/js/form.js”

In “/apps/dam/gui/coral/components/admin/metadataeditor/clientlibs/metadataeditor/js/form.js”, add ONLY lines 27-52 from following snippet. Please note the placement relative to an existing function “allCheckbox.each(function (index, checkbox))”